One method used to alleviate the problem of mismatch between elements is Dynamic Element Matching (hereinafter referred to as DEM). Here, “mismatch” means errors attributable to variations in the overall performance of the circuit through the use of one of a plurality of constituent circuit elements that have the same constitution but contain manufacturing errors, generated noise levels, and other differences.
DEM is a technology in which the element to be used at any time is determined randomly or by a prescribed algorithm, and the duty ratios of the various elements are averaged to reduce mismatch.
FIG. 9 is a diagram illustrating the constitution of a DEM circuit the prior art.
DEM part 100 is a digital encoder which performs the following operation, for example: digital input signal IN0 of binary weight is input, and M parallel outputs equal in number to the number of elements are output, to effect a conversion into a digital code column with the same weighting for the various codes. Elements 101-1, 102-2, . . . 102-M are sequentially coupled to the M output nodes of DEM part 100.
The number of the selected or operated elements defines the characteristics (such as the output signal level) at various times of the circuit containing said DEM constitution. Consequently, the number of codes that select or operate the elements in the code column output from DEM part 100 (hereinafter to be referred to as the active codes) is important.
When DEM is performed, for example, when the level of input signal IN0 lasts for a prescribed time, only a few elements in a certain region are selected and operated repeatedly. As a result, deviation of the duty ratio occurs, and this is a main cause of mismatch.
In DEM part 100, assignment of the elements that output the active codes is determined such that the duty ratio of the elements is averaged while the active code number needed at the various times is held. The mismatch error is therefore reduced, as is the mismatch itself.
In addition to the simple randomizing method, DEM mismatch can be reduced by storing the duty history of a certain range, and the active code is assigned preferably from those unused in said range.
The main method used to determine the DEM duty elements are primary mismatch reduction and secondary mismatch reduction constitutions. Here, the order represents the height of the level of the mismatch reducing function. For example, when the duty history is held as described above, the mismatch reduction performance is determined depending on how far back the stored duty history goes.
The secondary mismatch reduction constitution is very effective in suppressing noise. However, the secondary mismatch reduction constitution has the characteristic feature that the area increases dramatically as the number of elements increases. Consequently, when there are a large number of elements, the assembly area becomes larger, which is undesirable.
On the other hand, the primary mismatch reduction constitution allows assembly on a small area, but the performance is worse, and when there are elements with significant mismatch, the noise suppression performance is insufficient.
Consequently, the mismatch reduction method using DEM is a trade-off between the noise suppression performance and assembly area.
Consequently, it is difficult to improve the noise suppression performance while reducing the assembly area of the digital encoder with the DEM constitution shown in FIG. 9.
One purpose of the present invention is to propose a method that can increase the design freedom so as to relax the trade-off between assembly area and the noise suppression performance, and to improve the noise suppression performance while reducing the assembly area.
Another purpose of the present invention is to provide a digital-to-analog converter that can significantly reduce the error of the analog output signal and/or the assembly area by using said digital encoder.